The friction stir welding (FSW) process is being used in a number of heat exchanger applications to connect certain elements in the heat exchanger. For example, in the FSW process as applied to shell and tube heat exchangers, a solid state welding or stirring process is used wherein the ends of tube walls are “stirred” into surrounding tube sheet material without introducing dissimilar metals and without adverse effect to metal grain structure. Examples of the FSW process in heat exchanger applications are disclosed in U.S. Pat. No. 8,439,250 and in U.S. Published Patent Application No. 2013/0037601.
Because of the unique loading factors resulting from the FSW process, FSW has only been successfully applied to tubes with a thick gauge, for example greater than about 0.100 inch wall thickness. Overall, thin gauge tubes, for example about 0.100 inch wall thickness or less, can be more desirable for heat exchangers because they save material cost and decrease thermal resistance. However, the problem of employing FSW thin gauge tubes is difficult because the geometry of the FSW weld nugget can make it difficult to retain a satisfactory weld depth and can also increase the sensitivity and cost of pre-weld and post-weld machining processes.